The EDM process, now widely in use in industry, may be applied in various forms. For example, with "traveling-wire" EDM, a thin, continuous wire, tape or ribbon is employed as the tool electrode to form contours of various shapes in workpieces In "sinking38 EDM, a formed solid electrode may serve as the tool electrode to produce a cavity, which is complementary in shape thereto, in a workpiece. In "generic" EDM, a "non-formed" or generic electrode is constituted as the tool electrode moving in a scanning manner relative to the workpiece to form an intricate cavity or contour therein.
In the varying practical forms of EDM, it is a common requirement that the erosive current be furnished to the EDM gap as a succession of discrete, time-spaced current pulses, which can be created by pulsing a DC output by means of electronic switches or a capacitor. It has been found important that the current pulses be square or rectangular in waveform. For example, a workpiece must most often be "rough" machined. A "roughing" operation requires long current pulses whose duration or "on" time .tau.on ranges, say, in excess of 50 microseconds and often in the millisecond order. Given a particular mode of operation, it is important that "square" current pulses have a particular duration .tau.on, a particular pulse interval .tau.off and a particular current level Ip established in a particular combination to yield a particular set of machining results (e.g. surface roughness, overcut, relative electrode wear and stock removal) desired for that particular operation. Thus, the need arises to be capable of obtaining any "square" roughing-mode pulse with parameters .tau.on, .tau.off and Ip independently adjusted. This capability is indeed vital to a modern electroerosion generator as it is designed to apply a variety of rough and relatively rough machining operations.
With conventional EDM circuit arrangements, however, it has been found difficult to retain the squareness or rectangularity and the peak current of the EDM gap current pulses as desired. In these arrangements, the EDM pulses are produced in a unit (power supply or generator unit) provided separately from the machine proper which carries mechanical components and the work vessel in which the EDM gap is defined in the tool electrode and the workpiece. The separate power supply unit is adapted to package in its cabinet all principal electrical components required to produce at its output a succession of these pulses, the output of the unit being connected and hence the EDM pulses outgoing from the unit being transmitted to the machining gap by way of an elongated cable which must be provided. As a consequence, stray resistance and inductance included in the cable impede transmission of the shaped EDM pulses and act to cause a considerable loss of power and distortion of pulse waveform transmitted to the gap. Thus, there have been severe limitations in providing EDM pulses of greater Ip/.tau.on ratio.
These problems are even more serious in a "finish" machining operation required to achieve a finer EDM surface or to finish a rough EDM or otherwise machined workpiece. The finer the surface finish sought, the shorter must the pulse duration be, towards, and desirably into, the nanosecond order. Then, the greater the difficulty in retaining the squareness or rectangularity of erosive current pulses. Thus, when the pulse duration .tau.on must be as short as 10 microseconds or less, each pulse when delivered to the gap can at best be sinusoidal in its current-time characteristic, even when a most-advanced fine-pulsing erosion generator, either of switching or capacitor type, is employed. It has been found that the foregoing pulse transmission problems inherent in the prior art have severely hindered attempts to achieve improved results in the various ranges of the EDM process.